Required Navigation Performance

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Required Navigation Performance (RNP) is defined by ICAO as "a statement of the navigation performance necessary for operation within a defined airspace". Part of a broader concept called "Performance-based Navigation," RNP is a method of implementing routes and flight paths that differs from traditional methods in that an RNP path has an associated performance specification that an aircraft must meet before the path can be flown. Originally developed for trans-oceanic flight (where ground-based navigation infrastructure is non-existent), it has emerged in recent years as an important tool in the terminal area (airspace around airports). In that regard, it is significant because it greatly enhances the number of aircraft that can occupy a particular airspace without compromising safety.

Although technically not required, most RNP navigation is based on satellite navigation (e.g. GNSS). Before the introduction of the RNP concept, aircraft were assumed to be navigating using ground-based navaids, and the degree to which they follow an assigned track was based on the accuracy and signal propagation qualities of those ground-based systems. Assumptions about the precision to which altitude and airspeed could be controlled were based on pilots hand flying with reference to conventional instrumentation which has not changed in decades. These assumptions on the accuracy and precision to which an aircraft could fix its position in three dimensions in turn drove the size of the protected airspace which needed to be maintained around each instrument flight rules (IFR) aircraft. On many densely traveled routes, such as those over the northern Atlantic Ocean connecting the United States and Europe, aircraft were packed as tightly as the separation rules allowed during peak times (typically 30-50 nautical miles), thus limiting the number of flights which could travel that route per day.

RNP changes those assumptions. Under RNP, the nature of the navigational aids is not specified, rather the volume of airspace around the aircraft is, and this volume is smaller (in some cases much smaller) than that of conventional navigation. In practice, the RNP aircraft is assumed to be navigating using a combination of ground-based navaids, GPS, and inertial guidance systems, which give far greater precision that previously possible. This allows air traffic control to reduce the spacing between aircraft without compromising safety. Certain blocks of airspace are being designated with RNP standards; only aircraft meeting the designated RNP level for that airspace will be allowed to operate in that area.

In order to use RNP, equipment aboard the aircraft must be able to determine how well the navigation system is "performing," and furthermore, it must alert the flight crew whenever the performance degrades beyond the level required by the procedure being flown. This requirement for monitoring and alerting is what distinguishes RNP from RNAV, and is also what allows more precise paths with equivalent satellite signals.

The performance required to fly an RNP route is generally specified in nautical miles, e.g., RNP 4, and the airspace is protected at twice that value either side of centerline. So, an RNP 4 route protects 8 miles either side of centerline which means, in theory, that routes could be 16 nm on center. In the terminal area, RNP values of 0.30 nm are more typical, and there are RNP 0.10 nm approaches and departures in use today.

[edit] References

  • FAA Roadmap for Performance-Based Navigation Version 2.0 Final Draft
  • FAA Advisory Circular AC 90-101 Approval Guidance for RNP Procedures with SAAAR. Current guidance for air carriers wishing to conduct "Public SAAAR" procedures.
  • FAA Order 8260.52 United States Standard for Required Navigation Performance (RNP) Approach Procedures with Special Aircraft and Aircrew Authorization Required (SAAAR), TERPS order specifying criteria to be used in the construction of "Public SAAAR" RNP procedures.
  • FAA Advisory Circular AC 120-29A Criteria for Approval of Category I and Category II Weather Minima for Approach, see Appendix 5 (large file). Foundation for RNP operations in the terminal area.
  • FAA Notice N 8000.326 Guidelines for Airworthiness and Operational Approval and Procedure Design for Non-14 CFR PART 97 RNP SAAAR Approach Procedures, guidance for construction and approval of "Special SAAAR" RNP procedures (previously 8000.300 and 8000.287).
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